Production of cross-linked elastomeric yarns by dry spinning



United States Patent O 3,535,415 PRODUCTION OF CROSS-LINKED ELASTOMERIC YARNS BY DRY SPINNING Arnoldus Johannes Ultee, Afton, Va., assignor to E. I. du Pont de Nemonrs and Company, Wilmington, Del., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 416,122, Dec. 4, 1064. This application June 27, 1968, Ser. No. 740,481

Int. Cl. D01d 7/04; C08g 41/00 US. Cl. 264-205 8 Claims ABSTRACT OF THE DISCLOSURE CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of co-pending application Ser. No. 416,122, filed Dec. 4, 1964.

BACKGROUND OF THE INVENTION This invention relates to a process for preparing elastic yarns. More particularly, the invention relates to the preparation of cross-linked elastic yarns from solutions of substantially linear elastomers.

Linear spandex fibers having good elastic properties are known. Some of these products, however, are deficient in form stability when exposed under stretch to elevated temperature, especially in the presence of moisture. Under these conditions, they may shown excessive creep or set as judged by increased length in the stressed condition or after stress release, respectively. This also causes loss in retractive force at a constant use extension. The phenomenon is also known for segmented elastomers which do not belong to the class of segmented polyurethanes, and in case of compositions with lower melting hard segments makes them unsuitable for use as elastic fibers in garments exposed to laundering and hot-air drying. Finally, linear elastomers lacking hard segments altogether will flow or creep even at room temperature.

It is known that set and creep of elastomers in general can be reduced by cross-linking (vulcanization), but vulcanization processes as normally carried out require times of the order of an hour, at temperatures in the 140 C. range, which is entirely too slow for economic processing of fibers on the run. In case of thermal cure of spandex fibers with peroxides, as described in US. 3,154,611 (Dinbergs), convection cures are carried out at temperatures of 125175 C. for periods of at least one hour. The same patent describes faster cures at higher temperatures by contact heating and conduction transfer, but it is apparent from the examples that the temperatures used, up to 235 0., apply to the heated plates rather than to the fibers, and that a temperature gradient existed in the fibers allowing at least part of the fibers to remain in the solid state and support the threadline during this operation. This procedure would therefore not work with segmented elastomers having a low-melting hard segment component, such as styrene/butadiene block copolymers, or with elastomers which do not have hard segments, such as more or less random acrylonitrile/butadiene copolymers. In these cases, fast curing must be carried out in the molten threadline, using convection heat transfer from a fluid medium. The preferred procedure for this kind of curing would be a dry-spinning operation in which hot gas is used not only to evaporate the spinning solvent but also to effect cross-linking of the resulting filaments. Practical spinning speeds require this cure to be completed in one second or less. When applied to polyester-based segmented polyurethanes, as used as the examples of US. 3,154,611, only partial cures are obtained under these conditions. With polyether-based segmented polyurethanes, the polymer radicals are apparently more reactive, but chain-scission reactions compete with cross-linking, and no useful cured products can be obtained by use of peroxides alone.

SUMMARY OF THE INVENTION This invention provides a process for producing crosslinked, elastic filaments by dry-spinning and causing the cross-linking reaction to take place in the spinning cell. This invention also reduces the cost of rubber thread by providing a process for fabricating conventional rubber threads at high productivity rates from raw materials of low cost.

The advantages of this invention are attained by a process of producing a cross-linked, elastomeric filament which comprises:

(a) extruding through a spinneret into a dry-spinning cell a solution containing:

(1) at least 5% by weight of a substantially linear, elastic polymer selected from polyether-based segmented polyurethanes and polymers and copolymers of dienes, and having a molecular weight of at least 10,000 and a second-order transition temperature below 25 C.,

(2) at least about 0.1% by weight based on said polymer of a free-radical generator having a half-life at 200 C. of less than five seconds, and

(3) at least about 1% by weight based on said polymer of a coagent selected from compounds having a plurality of vinyl groups and compounds having a plurality of maleimide groups, said coagent having a boiling point greater than C., with the proviso that no separate coagent need be used when the elastic polymer contains at least about one percent by weight of polydiene segments which exceed 1000 molecular weight;

(b) removing solvent from the extrudate by means of hot inert gas circulated through the dry-spinning cell to form a filament and heating the filament in the cell to a temperature of at least 180 C. for a period of less than one second to produce a cross-linked filament.

Solution concentrations are chosen to give spinnable viscosities for dry-spinning, for example, 30% solids and a viscosity of 1,000 poises at room temperature are satisfactory. The concentration and viscosity, of course, depend on the molecular weight of the linear elastomer and its interaction with the solvent.

The polymers which are useful as starting materials in the process of this invention are the polyether-based polyurethanes and polymers and copolymers from dienes. In addition, they should have a molecular weight of 10,000 or more, should have a substantially linear polymer structure, should have a second-order transition temperature below 25 C., and should be capable of being cross-linked. The term substantially linear is not intended to exclude polymers which have branches extending out from the main chain. Capability for cross-linking may be readily determined by a cross-linking test specified hereinafter.

Within the limitations given, the elastomers to which the process of this invention is applicable are generally those which respond to free-radical cross-linking in a conventional, bulk curing operation. Of course, the polymer must be soluble in a suitable spinning solvent so that solutions of suitable viscosity are obtained for shaping into threads. Typical linear elastomers which may be used are: segmented polyurethanes derived from bi-functional polyether intermediates, such as polytetramethylene ether glycol, polypropylene ether glycol, polybutylene formal glycol as well as copolymers derived from tetrahydrofuran, propylene oxide and similar cyclic oxides by ringopening polymerization; rubbery polymers of the 1,3- butadienes, particularly butadiene, 2-chlorobutadiene and isoprene, and both random and block copolymers thereof with other polymerizable compounds, such as styrene, vinylnaphthalene, vinylpyridine, acrylic acid, acrylonitrile, methyl acrylate, isobutylene, methyl vinyl ether, methyl vinyl ketone, and the like. Copolymers containing a polar monomer such as acrylonitrile are referred to below as polar polydienes.

The cross-linking test for determining whether a particular elastomer may be used in the process of this invention is carried out as follows. A sample of the prospective elastomer is dissolved in an appropriate solvent, as indicated below, to form a 10% solution. Dimethylacetamide is used as solvent for the segmented polyurethanes and nitrile rubbers and other polar polydienes, and xylene is used for the non-polar polydienes. To the solution is added 2.7% by weight of dicumyl peroxide and 2.7% by weight of m-phenylene-bismaleimide, based on the polymer. A film is cast from the solution on a glass plate. The solvent is evaporated at a temperature below 100 C. The film is carefully removed and pressed between plates at 140 C. for 45 minutes in a press. If the resulting film has a molecular weight between cross-link points (M of less than 10,000, as determined by conventional methods, the polymer in question is considered capable of being cross-linked.

Solvents which are useful in the present process for forming the spinning solutions are selected from the known solvents for the class of elastomer which is used. The solvent selected should give a spinnable solution of the linear elastomer, the prime requirements being that it should not interfere with the subsequent cross-linking reaction. For a dry-spinning operation, the solvent should have a boiling point preferably within the range from 100 to 200 C., preferably about 50 C. below the temperature to which the thread is subsequently heated for cross-linking. The solvent selected should produce a true solution of the polymer, not merely a dispersion of the polymer. Except for the above considerations, choice of the solvent is not critical. Useful solvents include hexamethylphosphorarnide, N-methylpyrrolidone, tetrahydrofuran, dimethylsulfoxide, and the dialkylamides, such as dimethylformamide and dimethylacetamide, for the segrnented polyurethane elastomers; dimethylacetamide and hexamethylphosphoramide for the nitrile rubbers and other polar polydienes; aromatic hydrocarbons, such as toluene and xylene, chlorinated hydrocarbons, such as chlorobenzene and trichloroethylene, and kerosene for the non-polar polydiene elastomers.

The free-radical generators are well known materials in the elastomer art. They are used in the present process at levels ranging from 0.1% up to about 5% by weight of active material based on the linear elastomer. Suitable free-radical generators include the peroxides, such as dicumyl peroxide and dibenzoyl peroxide; hydroperoxides, such as 2,5-dimethylhexane-2,S-dihydroperoxide; and the azodinitriles, such as azodiisobutyronitrile and azobisdimethylvaleronitrile.

The coagents used with the free-radical generators in the process of this invention are well known in the clastomer art. They are used in the present process in the range from about 1% to about 5% by weight, based on the linear elastomer. Suitable coagents include the maleimides of polyfunctional amines, such as m-phenylenebismaleimide; acrylamides and methacrylamides of polyfunctional amines, such as methylene bisacrylamide and triacrylylperhydrotriazine, acrylates and methacrylates of polyfunctional alcohols, such as tetramethylene dimethacrylate, diallyl, triallyl and polyallyl compounds, such as triallyl cyanurate and 1,2-polybutadiene.

In case the polymer itself contains at least 1% by weight of a polydiene structure, such as the polybutadiene segments of styrene/butadiene block copolymers, the use of a coagent becomes unnecessary since the polymer segments can function as such. This is only the case if the polydiene segments have a molecular weight of 1,000 or over, since more or less random copolymers of dienes with other monomers, e.g., butadiene/acrylonitrile c0- polymers of the nitrile rubber type, still require the use of a coagent for effective cross-linking during dry spinning according to the invention. Likewise, a polyether-based segmented polyurethane containing unsaturated end groups or pendant unsaturated groups along the chain cannot be effectively cross-linked by the process of the invention unless a coagent is present.

The solutions, containing linear elastomer, free-radical generator, and coagent as required, can be spun in accordance with the usual dry-spinning techniques by extruding them under pressure through spinneret orifices into a dry-spinning cell in which the solvent is evaporated by means of a hot, inert, gaseous medium. As soon as the solvent is removed from the shaped filaments they are heated to a temperature of at least C. for a very brief time in order to produce cross-linked, elastic filaments. The heating is accomplished on the running filaments in the spinning cell before the fibers leave the cell. It is preferable to use both a hot gaseous medium and radiant heat from a heated cell wall for purposes of heating the extruded filaments in order to achieve cross-linking. Heating may be done under reduced pressure but is preferably carried out in the presence of an inert gas in order to minimize the effects of oxidation. The temperature used should be high enough to accomplish the desired degree of cross-linking in less than one second. The temperature is selected so that the halflife for the free-radical generator is of the order of a few seconds or less. Preferably, this temperature is about 200 C.

Although the temperature of the extruded filaments in dry-spinning usually remains at the boiling point of the solvent until most of the solvent is removed, additional heat treatment by the hot cell gas is required to practice this invention in most cases. Accordingly, the yarn must heat up for a short time to a much higher temperature than the boiling point of the solvent and this is accomplished by use of high temperatures of the order of about 300 C. for the dry-spinning aspiration gas.

This invention will be further illustrated, but is not intended to be limited, by the following examples in which parts and percentages are by weight unless otherwise specified. The first example shows that simple extension of the peroxide curing procedure of US. 3,154,611 to higher temperatures and shorter times as obtainable in a dry-spinning operation does not lead to effectively cross-linked fibers.

EXAMPLE I Estane, the polyester urethane elastomer made by the B. F. Goodrich Company, is used for experiments involving dry-spinning with the addition of dicumyl peroxide. Analysis indicates that the Estane used is composed of soft segments based on 1,4-butanedio1 and adipic acid alternating with hard segments based on p,p'-methylenediphenyldiisocyanate ,(MDI) and 1,4-butanediol, the ratio of adipic acid units to MDI units in the polymer being 2.3.

Spin A A solution is made of 330 parts of Estane in 670 parts dimethylacetamide (33% solids) to which solution is added parts dicumyl peroxide. The mixture is heated to 75 C. then extruded at 22.5 grams per minute through a spinneret of 25 holes 0.125 mm. in diameter. The emerging filaments traverse a spinning cell 3.66 meters long heated to 240 C. Kemp gas heated to 240 C. is circulated co-currently through the cell to evaporate the solvent from the extruded filaments and to heat them above 200 C. The filaments are in the cell for 0.8 second. In the equipment used, the customary procedure is to pass the filaments through an air jet where they are false twisted and caused to coalesce inside the spinning cell as described in US 3,094,374. The coalesced yarn is then customarily passed over a series of rolls and guides and then packaged. In this spin, the filaments are so sticky that they cannot be run over the guides and rolls.

Spin B A solution is prepared of 950 parts Estane in 3590 parts dimethylformamide (21% solids) to which solution is added a slurry containing 48 grams each of titanium dioxide, poly(diethylaminoethyl methacrylate) and dicumyl peroxide in dimethylformamide to give a mixture containing 23% solids. This mixture is spun under conditions similar to those used in Spin A except that the spinneret contains 32 holes of 0.18 mm. diameter, the solution is extruded at a rate to give a yarn of 345 denier, and the cell is heated to 250 C. In spite of the higher cell temperature it is believed that the yarn is actually heated to a lower temperature than in Spin A because more solvent must be evaporated from the more dilute solution. When tested for cross-linking, it is found that 52% of the yarn is insoluble in hexamethylphosphoramide, and the insoluble portion swells to 25.7 times its weight in that solvent.

EXAMPLE II Polytetramethylene ether glycol having a molecular weight of about 2,000 and p,p'-methylenediphenyl diisocyanate are intimately mixed and reacted in the ratio of 2 mols of diisocyanate per mol of polyether glycol. The resulting isocyanate-terminated polymer (2,500 parts) containing 3.3% isocyanate groups is diluted with dimethylacetamide and reacted with a mixture of 30.88 parts of hydrazine and 5.11 parts of diethylamine to give a solution of segmented polyurethane having an inherent viscosity of 1.1 (measured at room temperature in hexamethylphosphoramide at a concentration of 0.5% To the solution of linear elastomer is added a slurry containing 125 parts of titanium dioxide and 125 parts of poly(diethylaminoethyl methacrylate) in dimethylacetamide containing dissolved therein 42 parts of dicumyl peroxide and 56 parts of m-phenylenebis'maleimide. The final mixture contains 33% solids and has a viscosity of 3,700 posies at room temperature. This mixture is heated to 90 C. and is extruded through a spinneret of 24 holes 0.15 millimeter in diameter. The emerging filaments traverse a spinning cell 3.66 meters long which is heated to 240 C. Kemp gas, essentially a mixture of about 87% nitrogen and 13% carbon dioxide, heated to 292 C., is circulated co-currently through the cell to evaporate the solvent from the extruded filaments and to heat them above 200 C. The filaments are in the cell for 0.96 second. Yarn of 232 denier is wound up from the cell at a rate of 250 yards per minute (228.6 meters per minute). The major portion ofthe yarn is insoluble in hexamethylphosphoramide, in which it swells to five times its weight. The yarn displays a retractive force of 0.119 gram per denier at 200% elongation.

Another sample of yarn is spun under the same conditions from the same spinning mixture, except that no dicumyl peroxide and no m-phenylenebismaleimide is present. Only a minor amount of this product is insoluble in hexamethylphosphoramide. The insoluble portion swells to more than 30 times its weight. This yarn shows a retractive force of 0.089 gram per denier at 200% elongation.

6 EXAMPLE III A solution containing one part of a copolymer of butadiene and acrylonitrile (Paracril BLT) in four parts of dimethylacetamide is prepared. The solution has a viscosity of 750 poises at room temperature. To the solution are added 0.02 part of dicumyl peroxide and 0.02 part of m-phenylenebismaleimide in 0.08 part of dimethylacetamide. The solution is dry spun under the same conditions as described in Example II to give an elastic yarn of 180 denier which displays a retractive force of 0.012 g.p.d. at 200% elongation. In a similar preparation containing the above ingredients except for the dicumyl peroxide and m-phenylenebismaleimide, the product is sticky and displays a retractive force of only 0.004 g.p.d. at 200% elongation.

EXAMPLE IV A solution is prepared from equal parts of xylene and Kraton 102 a block copolymer of butadiene and styrene, containing about 70% butadiene as a central block or segment of more than 10,000 molecular weight), with the addition of 1.5 weight percent antioxidant (Irganox 1010 made by the Geigy Chemical Corp.) based on polymer. Part of this solution is dry spun as such, whereas another part is provided with 3% dicumyl peroxide (crystalline Dicup made by Hercules Corp.) before spinning. The viscosity of these solutions is 675 and 660 poises at room temperature, respectively. The solutions are heated to 40 C. and extruded through a spinneret of 8 holes 0.15 millimeter in diameter. The spinning cell and conditions are the same as in Example II, except that the gas temperature is 220 C., the time of the filament in the cell 0.8 second, and the denier of the yarn 75. The yarn spun without peroxide is 97% soluble in xylene, whereas, the major part of the yarn spun with added dicumyl peroxide is insoluble and swells to 13.7 times its weight in this solvent. When stretched to 300% elongation and heated in an air oven at a rate of about 10 C. per minute, the soluble yarn breaks at 55 C. The cross-linked yarn, however, does not break until a temperature of 180 C. is reached. The cross-linked yarn maintains a retractive force of about 7 mg./denier in the 105-165 C. temperature range versus 30 mg./ denier at room temperature.

EXAMPLE V To 96 parts of freshly distilled 2,4-tolylene diisocyanate stirred under nitrogen at 70 C. is added over a 2-hour period 48 parts of freshly distilled trimethylolpropane monoallyl ether (a product of the Celanese Corporation of America). After the 2-hour period the reaction product is heated briefly to C., and 129 parts of the product are transferred to a kettle containing 1,000 parts of polytetramethylene ether glycol having a molecular weight of 1,000. This mixture is heated for 3% hours at a temperature of 8090 C. to yield a hydroxyl-terminated, allyl-modified prepolymer. A mixture of 767 parts of this prepolymer and 127.8 parts of P,p'-methylenediphenyl diisocyanate is reacted for 1 hour at 82 C. The product containing 1.68% isocyanate groups is diluted With 449 parts of dimethylacetamide. A portion of this solution (1129 parts) is added to a solution containing 1327 parts of aqueous hydrazine hydrate and 1.64 parts of diethylamine in 2881 parts of dimethylacetamide to yield a solution of segmented polyurethane having a viscosity of 177 poises and containing 18.25% solids. Solvent is removed under vacuum until a solution having a viscosity of 610 poises and 22.3% solids is obtained. To 503 parts of this solution is added 3.14 parts of dicumyl peroxde and 2.69 parts of trisacrylylperhydrotriazine dissolved in 15 parts of dimethylacetamide. This solution is spun in a dry-spinning cell under the conditions described in Example II. A yarn having good elasticity is obtained. A major portion of the yarn is insoluble in hexamethylphosphoramide and swells to 6 times its weight.

The process of this invention involves the application of special cross-linking agents to the high-speed process of dry-spinning. The cross-linking reaction can be accomplished during spinning times of less than one second without degradation or scorch from excessive heat. The cross-linking agents are chosen to react only at elevated temperatures so that the spinning solutions remain stable during storage at room temperature and can be handled by conventional, solution-spinning techniques without premature gelation. The cross-linking process of this invention is applicable to all polyether-based spandex compositions, requiring only the addition of a small amount of cross-linking agents and little or no modification of spinning equipment. It is also applicable to other segmented elastomers and polydienes from which no useful elastic fibers can be obtained without cross-linking.

I claim:

1. Process for producing a cross-linked, elastomeric filament which comprises:

(a) extruding through a spinneret into a dry-spinning cell a solution containing:

(1) at least by weight of a substantially linear, elastic polymer selected from polyetherbased segmented polyurethanes and polymers and copolymers of dienes, and having a molecular weight of at least 10,000 and a secondorder transition temperature below 25 C.,

(2) at least about 0.1% by weight based on said polymer of a free-radical generator having a half-life at 200 C. of less than five seconds, and

(3) at least about 1% by weight based on said polymer of a coagent selected from compounds having a plurality of vinyl groups and compounds having a plurality of maleimide groups, said coagent having a boiling point above 150 C., with the proviso that no separate coagent need be used when the elastic polymer contains at least about 1% by weight of polydiene segments of molecular weight greater than 1,000, and

(b) removing solvent from the extrudate by means of hot inert gas circulated through the dry-spinning cell to form a filament and heating the filament in the cell to a temperature of at least 180 C. for a period of less than one second to produce a crosslinked filament.

2. Process according to claim 1 wherein the linear elastic polymer is a polyether-based segmented polyurethane.

3. Process according to claim 1 where the linear elastic polymer is a polydiene or a copolymer containing at least mole percent of a diene.

4. Process according to claim 1 wherein said freeradical generator is selected from the group consisting of peroxides, hydroperoxides, and azodinitriles.

5. Process according to claim 1 wherein the linear elastic polymer is a polyether-based segmented polyurethane, the free-radical generator is dicumyl peroxide, and the coagent is m-phenylenebismaleimide.

6. Process according to claim 1 wherein the linear elastic polymer is a polyether-based polyurethane, the freeradical generator is dicumyl peroxide, and the coagent is trisacrylylperhydrotriazine.

7. Process according to claim 1 wherein the linear elastic polymer is nitrile rubber, the free-radical generator is dicumyl peroxide and the coagent is m-phenylenebismaleimide.

8. Process according to claim 1 wherein the linear elastic polymer is a styrene/butadiene block copolymer and the free-radical generator is dibenzoyl peroxide or dicumyl peroxide.

References Cited UNITED STATES PATENTS 3,111,368 11/1963 Romano 264-205 3,113,934 12/1963 Grossman 264174 3,133,135 5/1964 Ogle 264182 3,154,611 10/1964 Dinbergs 264176 3,219,633 11/1965 Boussu 260 3,265,672 8/ 1966 Pariser et al. 26079.7 3,354,251 11/1967 Thoma et al 264210 3,365,412 1/1968 Thoma et al.

3,365,526 1/1968 Wieden et al. 264-205 FOREIGN PATENTS 939,196 10/ 1963 Great Britain.

OTHER REFERENCES Kovacic et al.: Cross Linking, Journal of Amer. Chem. Soc., vol. 81, pp. 11901194.

JULIUS FROME, Primary Examiner A. H. KOECKERT, Assistant Examiner US. Cl. X.R. 

